This study reports on the effect of different levels of proficiency on the students' achievements in collaborative learning instruction among 30 Persian-speaking EFL college students. Having been divided into dyads with different levels of proficiency, these subjects participated in nine sessions of collaborative instruction based on the form-focused collaborative text-editing task. The results of this study are obtained through the analyses of the proficiency gains in formal aspects of language in text-editing pre and post-tests and text-reconstruction post-test. The study highlights the significant effect of this type of instruction on more proficient students. Reasons provided in this study are the language analytic ability of the students, learning styles and strategies, and motivation. The findings of this study are helpful to teachers of the classes with heterogeneous language proficiency. (Contains 3 figures, 3 tables and 2 notes.)